ZfD4 NAL icons disappear from Taskbar

WinXP SP2
Client 4.91.5
ZfD4

AOT objects are associated via C1 and to be displayed as icons on users' Taskbar.
Icons show up fine upon log-in, but disappear after some time and requires a reboot to get them back.
Clearing NALCache and Refreshing the NAL Launcher doesn't help.
Thanks.